Join two tables with conditions depending on multiples columns - sql-server

Question

In SQL Server 2008, I want to join two table on key that might have duplicate, but the match is unique with the information from other columns.
For a simplified purchase record example,
Table A:
UserId PayDate Amount
1 2015 100
1 2010 200
2 2014 150
Table B:
UserId OrderDate Count
1 2009 4
1 2014 2
2 2013 5
Desired Result:
UserId OrderDate PayDate Amount Count
1 2009 2010 200 4
1 2014 2015 100 2
2 2013 2014 150 5
It's guaranteed that:
Table A and Table B have same number of rows, and UserId in both table are same set of numbers.
For any UserId, PayDate is always later than OrderDate
Rows with same UserId are matched by sorted sequence of Date. For example, Row 1 in Table A should match Row 2 in Table B
My idea is that on both tables, first sort by Date, then add another Id column, then join on this Id column. But I not authorized to write anything into the database. How can I do this task?

Answer 1

Row_Number() will be your friend here. It allows you to add a virtual sequencing to your resultset.
Run this and study the output:
SELECT UserID
, OrderDate
, "Count" As do_not_use_reserved_words_for_column_names
, Row_Number() OVER (PARTITION BY UserID ORDER BY OrderDate) As sequence
FROM table_b
The PARTITION BY determines when the counter should be "reset" i.e. it should restart after a change of UserID
The ORDER BY, well, you've guessed it - determines the order of the sequence!
Pull this all together:
; WITH payments AS (
SELECT UserID
, PayDate
, Amount
, Row_Number() OVER (PARTITION BY UserID ORDER BY PayDate) As sequence
FROM table_b
)
, orders AS (
SELECT UserID
, OrderDate
, "Count" As do_not_use_reserved_words_for_column_names
, Row_Number() OVER (PARTITION BY UserID ORDER BY OrderDate) As sequence
FROM table_b
)
SELECT orders.UserID
, orders.OrderDate
, orders.do_not_use_reserved_words_for_column_names
, payments.PayDate
, payments.Amount
FROM orders
LEFT
JOIN payments
ON payments.UserID = orders.UserID
AND payments.sequence = orders.sequence
P.S. I've opted for an outer join because I assumed that there's not always going to be a payment for every order.

Answer 2

Try:
;WITH t1
AS
(
SELECT UserId, PayDate, Amount,
ROW_NUMBER() OVER (PARTITION BY UserId ORDER BY PayDate) AS RN
FROM TableA
),
t2
AS
(
SELECT UserId, OrderDate, [Count],
ROW_NUMBER() OVER (PARTITION BY UserId ORDER BY OrderDate) AS RN
FROM TableB
)
SELECT t1.UserId, t2.OrderDate, t1.PayDate, t1.Amount, t2.[Count]
FROM t1
INNER JOIN t2
ON t1.UserId = t2.UserId AND t1.RN = t2.RN